Origin of life update

The amino acid alanine

L = left handed

D = right handed

Sci Tech Daily writes about research on why most biological molecules are left handed and the relevance of that to the evolution of life:

Paradoxical Mechanisms Uncovered – Chemists 

Have Filled a Major Gap in the Origin of Life

Molecules often have a structural asymmetry called chirality, which means they can appear in alternative, mirror-image versions, akin to the left and right versions of human hands. One of the great mysteries about the origins of life on Earth is that virtually all of the fundamental molecules of biology, such as the building blocks of proteins and DNA, appear in just one chiral form.

Scripps Research chemists, in two high-profile studies, have now proposed an elegant solution to this mystery, showing how this single-handedness or “homochirality” could have become established in biology.

The studies were published in the Proceedings of the National Academy of Sciences on February 5, 2024, and in Nature on February 28, 2024. Together, they suggest that the emergence of homochirality was due largely to a chemistry phenomenon called kinetic resolution, in which one chiral form becomes more abundant than another due to faster production and/or slower depletion.

“There have been many proposals for how homochirality emerged in specific molecules—specific amino acids, for example—but we really have needed a more general theory,” says Donna Blackmond, Ph.D., professor and John C. Martin Chair in the Department of Chemistry at Scripps Research, who led both studies.

Missing from this body of work, however, has been a plausible prebiotic theory for the emergence of homochirality.

“There has been a tendency in the field to ignore the chirality issue when looking for plausible reactions that could have made the first biological molecules,” Blackmond says. “It’s frustrating, because without reactions that favor homochirality, we wouldn’t have life.”

Ordinary chemical reactions that produce chiral molecules tend to yield equal (“racemic”) mixes of left- and right-handed forms. Outside of biology, this mixing typically doesn’t matter, as both forms usually have similar or identical properties. Within biology, though, as a consequence of extensive homochirality, it is commonly the case that only the left- or the right-handed form of a chiral molecule has useful properties—the other may be inert or even toxic. Thus, cells often guide reactions to yield specific chiral forms, using highly evolved enzymes.

The prebiotic Earth would not have had such enzymes, though—so how did homochirality ever arise?

In their study in Proceedings of the National Academy of Sciences, Blackmond and her team addressed this problem for amino acids. These small organic molecules are used as building blocks for proteins by all living things on Earth, but exist in biology in just the left-handed chiral form.

The researchers specifically sought to reproduce homochirality in a central process in amino acid production called transamination, by using a relatively simple, plausibly prebiotic chemistry that excludes complex enzymes.

In early tests, the team’s experimental reaction worked, and yielded amino acids that were enriched for one chiral form versus the other. The problem was that the favored form was the right-handed form—the one that biology doesn’t use.

“We were stuck for a while, but then the light bulb went on—we realized we could do part of the reaction in reverse,” Blackmond says.

When they did that, the reaction no longer preferentially made right-handed amino acids. In a striking example of kinetic resolution, it instead preferentially consumed and depleted the right-handed versions—leaving more of the desired left-handed amino acids. It thus served as a plausible route to homochirality for amino acids used in living cells.

Mirror images of alanine

Note the central carbon atom with sp3 orbitals:

It forms four bonds in the shape of a pyramid

So over time, the slow to make left-handed molecules accumulated because the faster to make right handed ones were less stable. 

By Germaine: Interested in science stuff

Carbon can form bonds in sp, sp2 or sp3 configurations

sp = linear bonding

sp2 = flat triangle bonding

sp3 = 3-dimensional pyramid bonding